1. Field of the Invention
The present invention generally relates to epitaxial growth and more particularly to an improved method and apparatus for forming a desired epitaxial layer on a crystalline substrate so as to provide desired semi-conductors of high quality, particularly of the light-emitting type.
2. Prior Art
Epitaxial layers of crystalline semi-conductive material have been formed on crystalline substrates by various techniques in the preparation of semi-conductors.
One such widely used technique is known as solution growth or liquid phase epitaxy. It generally comprises exposing the surface of a crystalline substrate to a solution of semi-conductive material dissolved in a molten metallic solvent, then cooling the solution sufficiently to cause supersaturation and a portion of the dissolved semi-conductive material to precipitate on the substrate as an epitaxial layer, after which the remaining solution is removed, as by decanting, from the substrate. The epitaxial crystalline growth layer may be of a single given conductivity type or of mutually opposite types. Successive epitaxial layers of mutually opposite types can also be deposited from solutions of mixed semi-conductive materials, but this has been difficult to do in a controlled, and reproducible manner. Light-emitting devices, such as diodes, can be formed utilizing liquid phase epitaxy but numerous processing requirements must be met, depending on the particular metallurgy of the compounds of Groups III-V of the Periodic Table selected and used in the processing, their eutectic temperature ranges, diffusion constants, lattice match, etc.
Numerous attempts have been made to render the liquid phase epitaxy technique more reliable and suitable for mass production of light-emitting devices. One prominent process is based on the lateral movement of a melt over the surface of the substrate wafer to deposit a given amount of the metallic solvent from a supersaturated solution on the wafer surface. This process has been described by a number of authors and is mostly a variant of the methods published by I. Hayashi, M. B. Panish, and P. W. Foy. (IEEE J. Quantum Electron. QE-5, 211, 1969) and M. B. Panish, I. Hayashi and S. Sumski, (Appl. Phys. Letters, Vol. 16, No. 8, April 15, 1970, pp 326.)
Such methods are based on the original Nelson U.S. Pat. No. 3,158,512 (1964), in which a wafer was brought into contact with the saturated melt by a tipping method, or its variant as set forth in Panish U.S. Pat. No. 3,557,219 (1970) and U.S. Pat. No. 3,560,276 (1971) among others. A similar method was patented by M. R. Corenz et al. in U.S. Pat. No. 3,540,941 (1970). The tipping process, however, needs a complex movement of the epitaxial heating zone or oven arrangement and was later replaced by a lateral movement of the melt and wafers in a graphite holder within the oven. Such processes were patented also by H. Nelson under U.S. Pat. No. 3,565,702 (1971) and were used extensively. However, such processes do not sufficiently limit the melt or protect the wafer surface against melt-clustering and so have been found to be unsatisfactory for many commercial applications.
Newer tipping methods have been designed in which vertically arranged wafers are introduced into a melt (e.g., L. E. Stone, K. Madden, and K. W. Haisty, "Growth of thick Ga.sub.x Al.sub.1-x as layers by liquid-phase epitaxy" in Journal of Electronic Materials, No. 1, 1972). The drawback in these methods is that the wafer is not subjected to the correct temperature gradient during the cooling cycle and that large size melts are needed. Further refinements of these methods were published, e.g. by J. M. Blum et al (Proceed. IEEE, Vol. 59, No. 10, October 1971). These authors improved on the original methods by transporting a small saturated melt from the saturation source directly to the substrate.
A more refined treatment was published by A. A. Berg, R. H. Saul, and C. R. Paola (Growth of GaP-wafers from thin aliquot melts: "Liquid Phase Epitaxy as a Commercial Process", J. Electrochemical Society, Vol. 120, No. 11, November 1973, pp. 1558-1563). Here, for the first time, it was clearly recognized that the deposition efficiency decreases with the melt thickness and that an aliquot melt is important to prepare the substrate surface for the actual crystallization process. It is also recognized that a larger melt prepared with the correct stoichiometric ratio and dopants, homogenized at high temperature, has to be brought in contact with the substrate. The problem with the processes working in horizontal arrangements is the lack of a temperature gradient vertical to the substrate melt sandwich. This gradient is important during the annealing period for correct and faultless outgrowth of the expitaxial layer.
Various arrangements of surface heaters in horizontal ovens have been employed in an effort to obtain proper temperature gradients and outgrowth of epitaxial layers using newer processing steps. However, they have met with little success because it is difficult to adapt the vertical gradient to the cooling cycle. (W. G. Rado, W. J. Johnson, and R. L. Crawley, J. Appl. Phys., Vol. 43, No. 6, June 1972, pp. 2763-2765.)
Accordingly, there still is a need for an improved method and apparatus to assure correct and faultless outgrowth of the epitaxial layer for the preparation of reliable superior III- V semi-conductor junctions for light-emitters in a manner which can be used commercially.